Four color modular printhead system

ABSTRACT

A printhead system includes a plurality of printhead assemblies ( 10 ) aligned in end-to-end relationship. Each printhead assembly ( 10 ) includes a plurality of printhead modules ( 12 ), the printhead modules ( 12 ) being arranged in end-to-end relationship and being angled with respect to a longitudinal axis of the assembly ( 10 ) such that printhead chips ( 18 ) of adjacent modules ( 12 ) overlap in a direction transversely to a direction of movement of print media past the assemblies ( 10 ).

FIELD OF THE INVENTION

[0001] This invention relates to a printhead assembly. Moreparticularly, the invention relates to a pagewidth inkjet printheadassembly.

SUMMARY OF THE INVENTION

[0002] According to the invention there is provided a printhead systemwhich includes a plurality of printhead assemblies aligned in end-to-endrelationship, each printhead assembly including a plurality of printheadmodules, the printhead modules being arranged in end-to-end relationshipand being angled with respect to a longitudinal axis of the assemblysuch that printhead chips of adjacent modules overlap in a directiontransversely to a direction of movement of print media past theassemblies.

[0003] The printhead module at one end of each assembly may have aprojecting portion which projects beyond an end of its assembly and theprinthead module at the other end has a recessed portion to receive theprojecting portion of the printhead module at said one end of anadjacent assembly.

[0004] The printhead module may comprise a microelec-tromechanicalprinthead chip comprised of a number of inkjet nozzles, the nozzles ofoverlapping portions of adjacent modules to be used being digitallyselected.

[0005] The angle of the printhead modules relative to the longitudinalaxis of the assembly may be selected depending on a print patternrequired. Each printhead module may have approximately 1587 dots perinch (dpi). To simulate 1600 dpi printing the printheads may be angledat approximately 7° to the longitudinal axis, more specifically 7.17°.

[0006] Each assembly may include a chassis and an ink reservoir mountedon the chassis, the printhead modules of the assembly being attached tothe ink reservoir. Preferably, the modules are releasably attached tothe ink reservoir.

[0007] The assembly may include an ink supply system for supplying inkto the reservoirs of each assembly.

[0008] The chassis may be a rigid chassis for imparting torsionalrigidity to each assembly.

[0009] The ink reservoir of each assembly may have ink inlet nozzles atone end and sealable air bleeding openings at an opposed end

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The invention is now described by way of example with referenceto the accompanying drawings in which:

[0011]FIG. 1 shows a three dimensional view, from above, of a printheadassembly, in accordance with the invention;

[0012]FIG. 2 shows a three dimensional view, from below, of theassembly;

[0013]FIG. 3 shows a three dimensional, exploded view of the assembly;

[0014]FIG. 4 shows a bottom view of the assembly;

[0015]FIG. 5 shows a three dimensional view, from below, of the assemblywith parts omitted;

[0016]FIG. 6 shows, on an enlarged scale, an end view of the assembly;

[0017]FIG. 7 shows, on the enlarged scale, a sectional end view of theassembly:

[0018]FIG. 8 shows a three dimensional, exploded view of a printheadmodule of the assembly;

[0019]FIG. 9 shows a bottom view of the module;

[0020]FIG. 10 shows a plan view of the module;

[0021]FIG. 11 shows a sectional end view of the module taken along lineXI-XI in FIG. 10;

[0022]FIG. 12 shows a three dimensional, exploded view of an inkreservoir of the assembly;

[0023]FIG. 13 shows a three dimensional view of a flexible printedcircuit board of the assembly;

[0024]FIG. 14 shows a three dimensional, exploded view of a busbararrangement of the assembly;

[0025]FIG. 15 shows a three dimensional view of a multiple printheadassembly configuration; and

[0026]FIG. 16 shows, on an enlarged scale, a sectional side view of thebonding of the printhead chip to the TAB film.

DETAILED DESCRIPTION OF THE DRAWINGS

[0027] A printhead assembly, in accordance with the invention isdesignated generally by the reference numeral 10. The assembly 10 uses aplurality of replaceable printhead modules 12. The advantage of thisarrangement is the ability to easily remove and replace any defectivemodules 12 in the assembly 10. This eliminates having to scrap an entireprinthead assembly 10 if only one module 12 is defective.

[0028] The assembly 10 comprises a chassis 14 on which an ink reservoir16 is secured. The printhead modules 12 are, in turn, attached to thereservoir 16.

[0029] Each printhead module 12 is comprised of a microelectromechanical(Memjet) chip 18 (shown most clearly in FIG. 8 of the drawings) bondedby adhesive 20 to a Tape Automated Bond (TAB) film 22, the TAB film 22being electrically connected to the chip 18. The chip 18 and the TABfilm 22 form a sub-assembly 24 which is attached to a micromolding 26.The micromolding 26 is, in turn, supported on a cover molding 28.

[0030] Each module 12 forms a sealed unit with four independent inkchambers 30 defined in the cover molding 28, the ink chambers 30supplying ink to the chip 18. Each printhead module 12 is plugged into areservoir molding 32 (shown most clearly in FIGS. 3 and 7 of thedrawings) of the ink reservoir 16 that supplies the ink. Ten modules 12butt together into the reservoir 16 to form a complete 8 inch printheadassembly 10. The ink reservoirs 16 themselves are modular, so complete 8inch printhead arrays can be configured to form a printhead assembly 10of a desired width.

[0031] The 8 inch modular printhead assembly 10, according to theinvention, is designed for a print speed and inkflow rate that allows upto 160 pages per minute printing at 1600 dpi photographic quality.Additionally, a second printhead assembly, of the same construction, canbe mounted in a printer on the opposite side for double sided high speedprinting.

[0032] As described above, and as illustrated most clearly in FIG. 8 ofthe drawings, at the heart of the printhead assembly 10 is the Memjetchip 18. The TAB film 22 is bonded on to the chip 18 and is sealed withthe adhesive 20 around all edges of the chip 18 on both sides. Thisforms the core Memjet printhead chip sub-assembly 24.

[0033] The sub-assembly 24 is bonded on to the micromolding 26. Thismolding 26 mates with the TAB film 22 which, together, form a floor 34(FIG. 11) of the ink chambers 30 of the cover molding 28. The chambers30 open in a flared manner in a top 36 of the cover molding 28 to definefilling funnels 38. A soft elastomeric, hydrophobic collar 40 isarranged above each funnel 38. The collars 40 sealingly engage withcomplementary filling formations or nozzles 42 (FIG. 7) of the reservoirmolding 32 of the ink reservoir 16 to duct ink to the chip 18.

[0034] Snap details or clips 44 project from the top 36 of the covermolding 28 to clip the cover molding 28 releasably to the ink reservoir16.

[0035] The TAB film 22 extends up an angled side wait 46 of the covermolding 28 where it is also bonded in place. The side wall 46 of thecover molding 28 provides the TAB film 22 with a suitable bearingsurface for data and power contact pads 48 (FIG. 8).

[0036] The sub-assembly 24, the micromolding 26 and the cover molding 28together form the Memjet printhead module 12. A plurality of theseprinthead modules 12 snap fit in angled, end-to-end relationship on tothe ink reservoir 16. The reservoir 16 acts as a carrier for the modules12 and provides ink ducts 52 (FIG. 7) for four ink colors, Cyan,Magenta, Yellow and blacK (CMYK). The four ink colors are channelledthrough the individual funnels 38 of the cover molding 28 into eachprinthead module 12.

[0037] The printhead modules 12 butt up to one another in anoverlapping, angled fashion as illustrated most clearly in FIGS. 2 and 4of the drawings. This is to allow the Memjet chips 18 to diagonallyoverlap in order to produce continuous printhead lengths from 0.8 inchesto 72 inches (for wide format printers) and beyond.

[0038] The Memjet chip 18 is 21.0 mm long×0.54 mm wide and 0.3 mm high.A protective silicon nozzle shield that is 0.3 mm high is bonded to theupper surface of the Memjet chip 18.

[0039] Each Memjet nozzle includes a thermoelastic actuator that isattached to a moving nozzle assembly. The actuator has two structurallyindependent layers of titanium nitride (TiN) that are attached to ananchor on the silicon substrate at one end and a silicon nitride(nitride) lever arm/nozzle assembly at the other end. The top TiN or“heater” layer forms an electrical circuit which is isolated from theink by nitride. The moving nozzle is positioned over an ink supplychannel that extends through the silicon substrate. The ink supplychannel is fluidically sealed around the substrate holes periphery by aTiN sealing rim. Ink ejection is prevented between the TiN rim and thenitride nozzle assembly by the action of surface tension over a 1 microngap.

[0040] A 1 microsecond 3V, 27 mA pulse (85 nanojoules) is applied to theterminals of the heater layer, increasing the heater temperature byJoule heating. The transient thermal field causes an expansion of theheater layer that is structurally relieved by an “out of plane”deflection caused by the presence of the other TiN layer.

[0041] Deflection at the actuator tip is amplified by the lever arm andforces the nozzle assembly towards the silicon ink supply channel. Thenozzle assembly's movement combines with the inertia and viscous drag ofthe ink in the supply channel to generate a positive pressure field thatcauses the ejection of a droplet.

[0042] Memjet actuation is caused by a transient thermal field. Thepassive TiN layer only heats up by thermal conduction after dropletejection. Thermal energy dissipates by thermal conduction into thesubstrate and the ink, causing the actuator to return to the ‘at rest’position. Thermal energy is dissipated away from the printhead chip byejected droplets. The drop ejection process takes around 5 microseconds.The nozzle refills and waste heat diffuses within 20 microsecondsallowing a 50 KHz drop ejection rate.

[0043] The Memjet chip 18 has 1600 nozzles per inch for each color. Thisallows true 1600 dpi color printing, resulting in full photographicimage quality. A 21 mm CMYK chip 18 has 5280 nozzles. Each nozzle has ashift register, a transfer register, an enable gate, and a drivetransistor. Sixteen data connections drive the chip 18.

[0044] Some configurations of Memjet chips 18 require a nozzle shield.This nozzle shield is a micromachined silicon part which is wafer bondedto the front surface of the wafer. It protects the Memjet nozzles fromforeign particles and contact with solid objects and allows thepackaging operation to be high yield.

[0045] The TAB film 22 is a standard single sided TAB film comprised ofpolyimide and copper layers. A slot accommodates the Memjet chip 18. TheTAB film 22 includes gold plated contact pads 48 that connect with aflexible printed circuit board (PCB) 54 (FIG. 13) of the assembly 10 andbusbar contacts 56 (FIG. 14) of busbars 58 and 60 of the assembly 10 toget data and power respectively to the chip 18. Protruding bond wiresare gold bumped, then bonded to bond pads of the Memjet chip 18.

[0046] The junction between the TAB film 22 and all the chip sidewallshas sealant applied to the front face in the first instance. Thesub-assembly 24 is then turned over and sealant is applied to the rearjunction. This is done to completely seal the chip 18 and the TAB film22 together to protect electrical contact because the TAB film 22 formsthe floor 34 of the ink chambers 30 in the printhead module 12.

[0047] The flexible PCB 54 is a single sided component that supplies theTAB films 22 of each printhead module 12 with data connections throughcontact pads, which interface with corresponding contacts 48 on each TABfilm 22. The flex PCB 54 is mounted in abutting relationship with theTAB film 22 along the angled sidewall 46 of the cover molding 28. Theflex PCB 54 is maintained in electrical contact with the TAB film 22 ofeach printhead module 12 by means of a pressure pad 62 (FIG. 7). The PCB54 wraps underneath and along a correspondingly angled sidewall 64 ofthe ink reservoir molding 32 of the ink reservoir 16. The part of thePCB 54 against the sidewall 64 carries a 62 pin connector 66.

[0048] The sidewall 64 of the ink reservoir molding 32 of the inkreservoir 16 is angled to correspond with the sidewall 32 of the covermolding 16 so that, when the printhead module 12 is mated to the inkreservoir 16, the contacts 48 of the TAB film 22 wipe against those ofthe PCB 54. The angle also allows for easy removal of the module 12. Theflex PCB 54 is ‘sprung’ by the action of the deformable pressure pad 62which allows for positive pressure to be applied and maintained betweenthe contacts of the flex PCB 54 and the TAB film 22.

[0049] The micromolding 26 is a precision injection molding made of anAcetal type material. It accommodates the Memjet chip 18 (with the TABfilm 22 already attached) and mates with the cover molding 28.

[0050] Rib details 68 (FIG. 8) in the underside of the micromolding 26provide support for the TAB film 22 when they are bonded together. TheTAB film 22 forms the floor 34 of the printhead module 12, as there isenough structural integrity due to the pitch of the ribs 68 to support aflexible film. The edges of the TAB film 22 seal on the underside wallsof the cover molding 28.

[0051] The chip 18 is bonded on to 100 micron wide ribs 70 that run thelength of the micromolding 26. A channel 72 is defined between the ribs70 for providing the final ink feed into the nozzles of the Memjet chip18.

[0052] The design of the micromolding 26 allows for a physical overlapof the Memjet chips 18 when they are butted in a line. Because theMemjet chips 18 now form a continuous strip with a generous tolerance,they can be adjusted digitally to produce the required print pattern,rather than relying on very close tolerance moldings and exoticmaterials to perform the same function. The pitch of the modules 12 is20.33 mm.

[0053] The micromolding 26 fits inside the cover molding 28, themicromolding 26 bonding on to a set of vertical ribs 74 extending fromthe top 36 of the cover molding 28.

[0054] The cover molding 28 is a two shot, precision injection moldingthat combines an injected hard plastic body (Acetal) with softelastomeric features (synthetic rubber). This molding interfaces withthe sub-assembly 24 bonded to the micromolding 26. When bonded intoplace the base sub-assembly, comprising the sub-assembly 24 and themicromolding 26, mates with the vertical ribs 74 of the cover molding 28to form the sealed ink chambers 30.

[0055] As indicated above, an opening of each chamber 30 is surroundedby one of the collars 40. These soft collars 40 are made of ahydrophobic, elastomeric compound that seals against the ink nozzles 42of the ink reservoir 16. The snap fits 44 on the cover molding 28 locatethe module 12 with respect to the ink reservoir 16.

[0056] The ink reservoir 16 comprises the ink reservoir molding 32 and alid molding 76 (FIG. 7). The molding 32 is a simple four chamberinjection molding with the lid molding 76 that is bonded on top to forma sealed environment for each color ink. Ink supply pipes 78 (FIG. 12)are arranged at one end of the lid molding 76 to communicate with inkchannels 80 defined in the reservoir molding 32. Labyrinthine,hydrophobic air holes 82 are defined at an opposed end of the lidmolding 76. The air holes 82 are included for bleeding the channels 80during charging. These holes 82 are covered over with a self adhesivefilm 84 after charging.

[0057] The lid molding 76 has heat stakes 88, (pins that are designed tomelt and hold the molding onto another part) which position and securethe ink reservoir 16 to the punched, sheet metal chassis 14. Additionalheat stakes 90 are arranged along the reservoir molding 32. These stakesare shown after deformation in FIG. 1 of the drawings once the inkreservoir 16 has been secured to the chassis 14.

[0058] Receiving formations 92 are defined along the sides of thereservoir molding 32 for releasably receiving the clips 44 of theprinthead modules 12.

[0059] As previously described, the sidewall 64 on the side of thereservoir molding 32 provides a mounting area for the flexible PCB 54and data connector 66. The reservoir molding 32 also carries details forfacilitating the accurate mounting of the V− and V+ busbars 58 and 60,respectively.

[0060] The metal chassis 14 is a precision punched, folded and platedmetal chassis used to mount the printhead assembly 10 into variousproducts. The ink reservoir 16 is heat staked to the chassis 14 via theheat stakes 88 and 90. The chassis 14 includes a return edge 94 formechanical strength. The chassis 14 can be easily customized forprinthead mounting and any further part additions. It can also beextended in length to provide multiple arrays of printhead assemblies 10for wider format printers.

[0061] Slots 97 are defined in the chassis 14 for enabling access to begained to the clips 44 of the modules 12 to release the modules 12 fromthe ink reservoir 16 for enabling replacement of one or more of themodules 12.

[0062] Thin finger strip metallic strip busbars 58 and 60 conduct V− andV+, respectively, to the TAB film 22 on each printhead module 12. Thetwo busbars 58 and 60 are separated by an insulating strip 96 (FIG. 14).The flexible, finger-like contacts 56 are arranged along one side edgeof each busbar 58, 60. The contacts 56 electrically engage the relevantcontact pads 48 of the TAB film 22 of each module 12 for providing powerto the module 12. The contacts 56 are separated by fine rib details onthe underside of the ink reservoir molding 32.

[0063] A busbar sub-assembly 98, comprising the busbars 58, 60 and theinsulating strip 96 is mounted on the underside of the sidewall 64 ofthe reservoir molding 32 of the ink reservoir 16. The sub-assembly isheld captive between that sidewall 64 and the sidewall 46 of the covermolding 28 by the pressure pad 62.

[0064] A single spade connector 100 is fixed to a protrusion 102 on thebusbar 58 for ground. Two spade connectors 104 are mounted oncorresponding protrusions 106 on the busbar 60 for power. Thearrangement is such that, when the sub-assembly 98 is assembled, thespade connectors 104 are arranged on opposite sides of the spadeconnector 100. In this way, the likelihood of reversing polarity of thepower supply to the assembly 10, when the assembly 10 is installed, isreduced. During printhead module 12 installation or replacement, theseare the first components to be disengaged, cutting power to the module12.

[0065] To assemble the printhead assembly 10, a Memjet chip 18 is drytested in flight by a pick and place robot, which also dices the waferand transports individual chips 18 to a TAB film bonding area. When achip 18 has been accepted, a TAB film 22 is picked, bumped and appliedto the chip 18.

[0066] A slot in the TAB film 22 that accepts the chip 18 and has theadhesive 20, which also functions as a sealant, applied to the upper andlower surfaces around the chip 18 on all sides. This operation forms acomplete seal with the side walls of the chip 18. The connecting wiresare potted during this process.

[0067] The Memjet chip 18 and TAB film 22 sub-assembly 24 is transportedto another machine containing a stock of micromoldings 26 for placingand bonding. Adhesive is applied to the underside of the fine ribs 70 inthe channel 72 of the micromolding 26 and the mating side of theunderside ribs 68 that lie directly underneath the TAB film 22. Thesub-assembly 24 is mated with the micromolding 26.

[0068] The micromolding sub-assembly, comprising the micromolding 26 andthe sub-assembly 24, is transported to a machine containing the covermoldings 28. When the micromolding sub-assembly and cover molding 28 arebonded together, the TAB film 22 is sealed on to the underside walls ofthe cover molding 28 to form a sealed unit. The TAB film 22 furtherwraps around and is glued to the sidewall 46 of the cover molding 28.

[0069] The chip 18, TAB film 22, micromolding 26 and cover molding 28assembly form a complete Memjet printhead module 12 with four sealedindependent ink chambers 30 and ink inlets 38.

[0070] The ink reservoir molding 32 and the cover molding 76 are bondedtogether to form a complete sealed unit. The sealing film 84 is placedpartially over the air outlet holes 82 so as not to completely seal theholes 82. Upon completion of the charging of ink into the ink reservoir16, the holes 82 are sealed by the film 84. The ink reservoir 16 is thenplaced and heat staked on to the metal chassis 14.

[0071] The full length flexible PCB 54 with a cushioned adhesive backingis bonded to the angled sidewall 64 of the ink reservoir 16. The flexPCB 54 terminates in the data connector 66, which is mounted on anexternal surface of the sidewall 64 of the ink reservoir 16.

[0072] Actuator V− and V+ connections are transmitted to each module 12by the two identical metal finger strip busbars 58 and 60. The busbarsub-assembly 98 is mounted above the flex PCB 54 on the underside of thesidewall 64 of the ink reservoir molding 32. The busbars 58, 60 and theinsulating strip 96 are located relative to the ink reservoir molding 32via pins (not shown) projecting from the sidewall 64 of the inkreservoir molding 32, the pins being received through locating holes 108in the busbars 58, 60 and the insulating strip 96.

[0073] The Memjet printhead modules 12 are clipped into the overhead inkreservoir molding 32. Accurate alignment of the module 12 to thereservoir molding 32 is not necessary, as a complete printhead assembly10 will undergo digital adjustment of each chip 18 during final QAtesting.

[0074] Each printhead module's TAB film 22 interfaces with the flex PCB54 and busbars 58, 60 as it is clipped into the ink reservoir 16. Todisengage a printhead module 12 from the reservoir 16, a custom tool isinserted through the appropriate slots 97 in the metal chassis 14 fromabove. The tool ‘fingers’ slide down the walls of the ink reservoirmolding 32, where they contact the clips 44 of the cover molding 28.Further pressure acts to ramp the four clips 44 out of engagement withthe receiving formations 92 and disengage the printhead module 12 fromthe ink reservoir 16.

[0075] To charge the ink reservoir 16 with ink, hoses 10 (FIG. 3) areattached to the pipes 78 and filtered ink from a supply is charged intoeach channel 80. The openings 82 at the other end of the ink reservoircover molding 76 are used to bleed off air during priming. The openings82 have tortuous ink paths that run across the surface, which connectthrough to the internal ink channels 80. These ink paths are partiallysealed by the bonded transparent plastic film 84 during charging. Thefilm 84 serves to indicate when inks are in the ink channels 80, so theycan be fully capped off when charging has been completed.

[0076] For electrical connections and testing, power and dataconnections are made to the flexible PCB 54. Final testing thencommences to calibrate the printhead modules 12. Upon successfulcompletion of the testing, the Memjet printhead assembly 10 has aplastic sealing film applied over the underside that caps the printheadmodules 12 and, more particularly, their chips 18, until productinstallation.

[0077] It is to be noted that there is an overlap between adjacentmodules 12. Part of the testing procedure determines which nozzles ofthe overlapping portions of the adjacent chips 18 are to be used.

[0078] As shown in FIG. 15 of the drawings, the design of the modularMemjet printhead assemblies 10 allows them to be butted together in anend-to-end configuration. It is therefore possible to build a multipleprinthead system 112 in, effectively, unlimited lengths. As long as eachprinthead assembly 10 is fed with ink, then it is entirely possible toconsider printhead widths of several hundred feet. This means that theonly width limit for a Memjet printer product is the maximummanufacturable size of the intended print media.

[0079]FIG. 15 shows bow a multiple Memjet printhead system 112 could beconfigured for wide format printers. Replaceable ink cartridges 114, onefor each color, are inserted into an intermediate ink reservoir 116 thatalways has a supply of filtered ink. Hoses 118 exit from the undersideof the reservoir 118 and connect up to the ink inlet pipes 78 of eachprinthead assembly 10.

[0080] It will be appreciated by persons skilled in the art thatnumerous variations and/or modifications may be made to the invention asshown in the specific embodiments without departing from the spirit orscope of the invention as broadly described. The present embodimentsare, therefore, to be considered in all respects as illustrative and notrestrictive.

We claim:
 1. A printhead system which includes a plurality of printheadassemblies aligned in end-to-end relationship, each printhead assemblyincluding a plurality of printhead modules, the printhead modules beingarranged in end-to-end relationship and being angled with respect to alongitudinal axis of the assembly such that printhead chips of adjacentmodules overlap in a direction transversely to a direction of movementof print media past the assemblies.
 2. The system as claimed in claim 1in which the printhead module at one end of each assembly has aprojecting portion which projects beyond an end of its assembly and theprinthead module at the other end has a recessed portion to receive theprojecting portion of the printhead module at said one end of anadjacent assembly.
 3. The system as claimed in claim 2 in which theprinthead module comprises a microelec-tromechanical printhead chipcomprised of a number of inkjet nozzles, the nozzles of overlappingportions of adjacent modules to be used being digitally selected.
 4. Thesystem as claimed in claim 1 in which the angle of the printhead modulesrelative to the longitudinal axis of the assembly is selected dependingon a print pattern required.
 5. The system as claimed in claim 1 inwhich each assembly includes a chassis and an ink reservoir mounted onthe chassis, the printhead modules of the assembly being attached to theink reservoir.
 6. The system as claimed in claim 5 in which the modulesare releasably attached to the ink reservoir.
 7. The system as claimedin claim 5 which includes an ink supply system for supplying ink to thereservoirs of each assembly.
 8. The system as claimed in claim 5 inwhich the chassis is a rigid chassis for imparting torsional rigidity toeach assembly.
 9. The system as claimed in claim 5 in which the inkreservoir of each assembly has ink inlet nozzles at one end and sealableair bleeding openings at an opposed end.